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Tuned pipes and glow plugs - updated 1/14/05

Questions regarding glow plugs come up very frequently, and I often find myself copying my previous posts. A good 40-50% of the problems I see on a daily basis are caused by improper or fouled plugs. Maybe this will clear up some confusion.

Choosing a glow plug
The 2.5 and 2.5R require a standard long plug with a medium-hot to hot temperature range. A shorter plug will reduce compression, causing a loss of power and complicating tuning. To eliminate headaches, it's always best to run the proper plug.

Differences between the above plugs are negligible with 10-20% fuel. All will work great. However, I have found that engines running high-nitro fuel (>30%) perform exceptionally well with the 3232. Because it has a heavy-gauge element, its temperature range is cooler (albeit very slightly) than comparable plugs - ideal for fuels with high nitro content.

Contrary to popular belief, the O.S. A3 and O.S. #8 are not ideal for the 2.5. Both are short plugs. The McCoy MC-8 and McCoy MC-9 are the proper length and will usually work fine in a pinch, but are colder than preferred.

Crush gasket
New plugs include a copper washer to achieve a proper seal. They are designed to be installed with the concave side facing down, like an upside-down bowl. When the plug is tightened, the soft material flattens slightly and provides a strong seal. Never install the washer with the convex side down; it will deform and leak. It is also important to re-install a used washer the same direction as before; although it may look compressed, the side with the imprinted ring should always face up.

"Reading" a glow plug
By examining a used plug closely, you can determine how and why it failed. Here is a guide by the late, great Ron Paris:

Tuned pipes - how and why
The need for a tuned pipe arises because of an inescapable flaw in two-stroke engine theory. Sure, these engines have outstanding power-to-displacement ratios, but only at the cost of fuel efficiency; they are far more thirsty than their frugal four-stroke counterparts. The reason for this lies in the crankcase & sleeve porting. On all two-stroke engines, the intake port(s) and exhaust port are open simultaneously. (A rear-exhaust engine, such as the TRX 2.5, positions the central intake port and the exhaust port in diametric opposition, thus increasing the velocity of the gases slightly.) As raw fuel enters the intake port, a large amount passes through the combustion chamber and out the exhaust port without ignition. There’s no way to solve this problem without utilizing a four-stroke design, but a tuned pipe will do a great deal of good for both power and fuel efficiency. It uses a series of expanding and narrowing cones to take advantage of the escaping exhaust gases and force raw fuel back into the combustion chamber (we call this process scavenging). Best of all, using a tuned pipe has no tradeoff whatsoever – a rare thing indeed with engines.
In addition to enhancing efficiency, the pipe plays a vital role in fuel delivery. As the engine increases in speed, its need for fuel increases exponentially. Most of the fuel is provided by negative pressure pulses from the crankcase, but to compensate, the engine must also have a pressure nipple on the pipe. This connects to the fuel tank, and as exhaust pressures build with RPM, fuel delivery to the carburetor is accelerated. As such, a proper tuned pipe will deliver more power, better fuel economy, and longer engine life.

edit: I've received a few questions about this diagram. It represents a generic two-chamber pipe, not the OEM Revo part.

A) Header Adjusting the length of the header can make a substantial difference in the engine’s performance. A longer header will increase torque at the expense of throttle response and high-RPM power. A shorter header will increase power at higher RPM and speed throttle response, but decreases torque. B) Divergent cone This is where the tuned pipe action starts. Exhaust gases flow through the header and into the divergent cone. As the pipe widens, the gases slow considerably (Bernoulli's principle*). The slowing of the gases creates a low-pressure void behind the escaping charge, which sucks any remaining fuel vapor away from the exhaust port. A short divergent cone is good for torquey engines, whereas a long divergent cone is good for high-RPM applications.C) Belly The belly of the pipe isn’t as significant as the other components, but its shape and size still have a considerable effect on engine performance and tuning. A short belly is ideal for high-RPM, and a long belly is good for torque. Certain pipes may have a small baffle in the belly to assist in building & redirecting pressure; if you see one, don’t bother modifying or removing it.D) Convergent cone Now things get a bit more complicated. As the exhaust travels to the end of the belly of the pipe, most of it exits through the stinger (E). However, some strikes the convergent cone and begins to gain velocity as the pipe narrows (Bernoulli again). The pulse then reflects off the end of the pipe and flows back towards the header. The pulse returns the exhaust port (open at this point) and pushes any escaping fuel vapor back into the combustion chamber for re-ignition.E) “Stinger” I’ve never seen a proper name for this little bugger, but "stinger" seems to be the common consensus. So that it shall be. The inner diameter regulates the amount of air exiting the pipe – too small, and the engine will stifle itself; too large, and it will allow far too much exhaust to escape during the first cycle, negating the effects of the convergent cone altogether. On a .12-.15ci (2.0-2.5cc) engine, the stinger's inner diameter is usually about 5.5-6.5mm (~.25") depending on pipe design.F) Pressure fitting A small amount of pressure in the pipe is pushed through this fitting; it provides some backpressure in the fuel system, assisting in feeding the engine as RPMs increase. At higher engine speeds, exhaust pressure increases and pushes more fuel into the carb to compensate. Ideally, the fitting should be placed at the fattest section of the pipe belly, near where the two pressure waves converge.

So, what happens when you fail to use a tuned pipe? Since fuel delivery is regulated by the tuned pipe, without one tuning is impossible to perfect across the entire RPM range. As RPMs increase, the fuel mixture will progressively lean out, starving the engine of necessary lubrication and cooling - this makes the engine run more powerfully for a while, but quickly destroys it. Those “dual exhaust” systems sold on eBay are a death sentence for your engine. Although they all claim to have proper pressure characteristics, the science behind them is dangerous at best. No divergent cones (square “chambers” do nothing), no belly, and an improperly-placed fuel nipple all spell disaster. There’s a reason that no reputable engine manufacturer produces these things.

* Yes, I know that Bernoulli's principle isn't perfectly accurate when dealing with compressible fluids, but it's close enough. So there... the link is still cooler than sliced bread anyway.

Just to clarify a few things that BobR did not really touch on. The "Pulse he is referring to is actually a sound wave. Thus the "tune" in a tuned pipe! (Which is, in fact, a musical instrument.) Did that help at all?

In two stroke engines the sequence works like this. Bang! Combustion! The piston is forced down by the expanding gasses from the exploding fuel/air mixture. The exhaust port opens and the exhaust starts to exit. The fuel intake ports open and the positive pressure that exists in the crank case at that moment (This relates to the crank shaft inlet opening and closing and the movement of the piston but I am not covering that part here!) pushes fresh fuel/air into and the combustion chamber. The piston hits the bottom of the stroke and starts to move back up. Unburned fuel is now exiting with the exhaust. The fuel intake ports close as exhaust and fuel continue to exit. The exhaust port closes and the piston goes up to repeat the cycle. In the TRX 2.5 series engines this happens about 40,000 times per minute or 667 times per second at peak RPM!

The addition of a tuned pipe can take advantage of that fuel loss and use it to your advantage. The sound wave produced by the combustion of the fuel at the top of the stroke is released when the exhaust port opens. Once released it heads off at (amazingly enough) the speed of sound! The divergent cone, belly and convergent cone all act to direct and reflect the sound wave. The echo eventually (however long it takes a sound wave traveling at roughly 760 MPH to go about 14 inches) works its way back to the exhaust port which is still open! The intake ports have closed by this time which is important. The returning “pulse” reverses the direction of the exhaust flow and shoves the escaping unburned fuel back into the combustion chamber just before the exhaust port closes (scavenging what would otherwise have been exhaust residue on your chassis). You have now crammed more fuel into the combustion chamber than would normally be possible. BobR will explain the “Big Bang” theory to illustrate what happens next!
The timing of the returning sound wave is only correct within a specific RPM range. When you accelerate from a stop you can hear the engine “jump on pipe” when it hits that range. It sounds like you just shifted into another gear!

"Fortunately son, I says fortunately I keep my feathers numbered for just such an emergency!"
Foghorn Leghorn